Speaker A: Look, it's going to storm. Take my umbrella. Speaker B: ______

A．How will you go home if you give it to me

B．I have a raincoat in my office. Thanks anyway.

C．I think you need to use it yourself.

D．It doesn't matter. I will go without having your umbrella.

电机车换向磁极是用以（）。

A.电机的换向

B.减弱电机旋转时换向器上产生的换向火花

C.增加电机的牵引力

D.减少电机电流